This invention relates to a method for selectively ablating material from a workpiece by the use of laser beam radiation and more particularly to ablating metal workpieces by a pulse or pulses of laser beam radiation in which the cross-sectional intensity of the beam is temporally and spatially controlled.
The selective removal of the material by laser radiation is well known. However, these techniques are not efficient and the processes cannot be controlled with sufficient precision. Processes known to the prior art characteristically provide undesirable side effects. Such side effects involved smeared surfaces, cracks and the appearance of sputtered material on the surface of the workpiece around the ablated area, which material results from the noncomplete ejection of the metal during the ablation process.
Many prior art systems employ pulsed laser radiation for the ablation of material. Some laser systems produce a sequence of pulses having an irregular intensity distribution to cause the ablation. The cross-sectional intensity of the beam has not been temporally controlled by these systems.
Another method of ablation includes the use of laser pulses having varying lengths and intensities. For example, Swiss patent CH-PS No. 547,690 describes a system for generating a beam having a symmetrically uniform transverse spatial configuration. The beam is generated by an oscillator which provides single mode operation. The intensity distribution of the beam is of a Gaussian shape. However, such systems are generally unsatisfactory because the apparatus employed to carry out the processes are incapable of accurately reproducing the pattern.
Since most metal workpieces reflect a significant portion of the energy of the beam, undesirably high-powered systems are required to cause ablation. These high-powered systems are expensive and inefficient. However, German patent DOS No. 24 30 994 teaches that as the beam intensity increases, and as the heated surface interacts with the beam, the absorption level increases greatly and if the intensity of the beam is appropriately adjusted, the process can be efficient. However, this system and its process appear to be difficult to control because of the rapid changes in the absorption level at the workpiece.
Another proposed solution to the problem of ablation control is found in Swiss patent CH-PS No. 532,993 or U.S. Pat. No. 3,962,558. There the ablation of the material is effected incrementally by the application of successively applied spikes of energy from a pulsed laser. The first spike, which is of high intensity, appears to increase the absorption level of the workpiece and the smaller subsequently applied spikes provide a controlled vaporization of the metal workpiece. By determining the number of pulses applied to the workpiece, the depth of the ablation can be controlled. This process causes ablation through the successive vaporization of molten metal by each pulse without creating sufficient vapor pressure to overcome the surface tension of the molten liquid. Thus, no material is ejected in the area of impingement. The efficiency of the process, however, is limited because of the high amount of energy required to maintain the successive vaporization. If the intensity of the individual spikes provided to the workpiece is increased, the vapor pressure becomes sufficient to cause the ejection of the molten material. However, a significant amount of energy is lost in the ejected molten material at the area of impingement and, accordingly, each subsequently applied pulse must reheat and vaporize the material before it will again be ejected.
Theories for explaining the ablation process are not precisely established. However, it is generally believed that for beams having an intensity of less than 10.sup.7 W/cm.sup.2 at the surface of the workpiece, the ablating process seems to be limited by heat conduction losses. Hence, the heating and vaporization characteristics of the material are different for each type of material (e.g., steel, copper) ablated. However, as beam intensity increases, beginning with approximately 10.sup.9 W/cm.sup.2, a plasma forms in the area exposed to the beam at the workpiece surface and the efficiency is reduced because the increased energy is absorbed by the plume or the plasma rather than by the metal workpiece itself. This condition is not considered to be desirable.
Theories were also suggested which would attempt to predict the temperatures required to vaporize the material at the area of impingement, which temperatures were higher than the evaporation temperature of the material. Such temperatures tend to cause localized explosions in the workpiece at the area of impingement. Also, it has been determined that at particular beam intensities, portions of the ablated material in the area of impingement are not vaporized, but rather are ejected in molten fluid form. Areas between contiguous points of ablation on the workpiece were found to have deposited thereon material which had been ejected from the ablation. This, too, is an undesirable condition.
The present invention, therefore, provides a technique for ablating a metal workpiece with a laser beam operating at levels of efficiency higher than heretofore possible and an improved regulation of the ablation process. Moreover, the invention attempts to reduce the undesired side effects of the ablation process by eliminating the ejected material appearing in the areas between the contiguous points of ablation. The invention is the result of the discovery that the ablation process can be enhanced by the appropriate selection of beam intensity and by the appropriate selection of pulse shape.